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EP1276288B1 - Symboles de référence pour l'estimation de canal pour la transmission multiporteuse - Google Patents

Symboles de référence pour l'estimation de canal pour la transmission multiporteuse Download PDF

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Publication number
EP1276288B1
EP1276288B1 EP01116839A EP01116839A EP1276288B1 EP 1276288 B1 EP1276288 B1 EP 1276288B1 EP 01116839 A EP01116839 A EP 01116839A EP 01116839 A EP01116839 A EP 01116839A EP 1276288 B1 EP1276288 B1 EP 1276288B1
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EP
European Patent Office
Prior art keywords
ofdm
preamble
channel
data burst
pilot
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EP01116839A
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German (de)
English (en)
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EP1276288A1 (fr
Inventor
Ralf Advanced Technology Cent. Stuttgart Boehnke
Edgar Bolinth
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Sony Deutschland GmbH
Siemens AG
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Sony Deutschland GmbH
Siemens AG
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Priority to EP01116839A priority Critical patent/EP1276288B1/fr
Priority to ES01116839T priority patent/ES2278661T3/es
Priority to DE60125925T priority patent/DE60125925T2/de
Publication of EP1276288A1 publication Critical patent/EP1276288A1/fr
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/26—Systems using multi-frequency codes
    • H04L27/2601—Multicarrier modulation systems
    • H04L27/2602—Signal structure
    • H04L27/261—Details of reference signals
    • H04L27/2613—Structure of the reference signals
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L25/00—Baseband systems
    • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
    • H04L25/0202—Channel estimation
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/26—Systems using multi-frequency codes
    • H04L27/2601—Multicarrier modulation systems
    • H04L27/2614—Peak power aspects
    • H04L27/262—Reduction thereof by selection of pilot symbols
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/26—Systems using multi-frequency codes
    • H04L27/2601—Multicarrier modulation systems
    • H04L27/2602—Signal structure
    • H04L27/261—Details of reference signals
    • H04L27/2613—Structure of the reference signals
    • H04L27/26136—Pilot sequence conveying additional information

Definitions

  • the present invention generally relates to the field of wireless communication systems with high-speed mobile access, especially to wireless communication systems using reference symbols to support channel tracking in high-speed wireless multi-carrier systems.
  • a broadband radio channel as needed for the transmission of high data rates, is characterized by severe attenuation fades (frequency-selective fading) caused by the multipath propagation of the transmitted mobile radio signals.
  • severe attenuation fades frequency-selective fading
  • it exhibits a time-variant behavior due to the mobility of the receiver, which possibly requires a continuous adaptation of the transmission system to said behavior.
  • OFDM Orthogonal Frequency Division Multiplexing
  • W-ATM Wireless Asynchroncus Transfer Mode
  • DMT Asymmetric Digital Subscriber Lines
  • Multi-carrier modulation is also considered for use in high-speed modems, but so far all ITU-T (formerly CCITT) standards refer to single-carrier systems.
  • OFDM was initially standardized for the two high-speed digital radio transmission systems Digital Audio Broadcasting (DAB) and Digital Video Broadcasting Terrestrial transmission mode (DVB-T), which are used for transmitting digital radio and television signals. Both systems are real broadcast systems for transmitting data via broadband radio channels.
  • DAB Digital Audio Broadcasting
  • DVD-T Digital Video Broadcasting Terrestrial transmission mode
  • Both systems are real broadcast systems for transmitting data via broadband radio channels.
  • DAB was originally developed for mobile reception, it is able to transmit data rates of up to 1.7 MBit/s.
  • DVB-T more than 20 MBit/s can be transmitted -, however, the system was originally developed for stationary reception. Aside from video signal transmission, today additional data services are planned for DVB-T.
  • OFDM is a viable technology for high-speed data transmission over spectrally shaped noisy channels.
  • a further field of application, for which the OFDM transmission system plays an increasing role, is the mobile access to wire-bound networks with the aid of a local radio network, whereby high data rates shall also be transmitted.
  • the HIPERLAN/2 standard as well as an extension of the IEEE 802.11 standard for the 5 GHz area can be mentioned.
  • the OFDM transmission system is applied.
  • broadband single-carrier systems have been specified with the denotation Universal Mobile Telecommunications System (UMTS).
  • UMTS Universal Mobile Telecommunications System
  • a higher data rate would cause a very high expenditure during the channel equalization.
  • OFDM could also offer a sensible alternative for real mobile applications, and thus represent an important step for the mobile Internet.
  • ISI intersymbol interference
  • the transmitted data is split into a number of parallel data streams, each one used to modulate a separate subcarrier.
  • FDM Frequency Division Multiplex
  • different signals are independently generated and parallel transmitted using different frequency bands.
  • the channel spectrum is parsed into a large number of parallel subchannels which are, in the ideal case, independent and ideally frequency-nonselective.
  • the spectral subcarrier separation in the receiver is achieved by means of steep bandpass filters, resulting in a considerable amount of realization complextity, especially if the number of the employed subcarriers is very high.
  • OFDM is known to be an elegant solution for the severe ISI problem, as it avoids an equalization of the entire band.
  • OFDM transmission systems manage the above-described situation by dividing up the broadband radio channel into many narrow-band subchannels. Instead of converting a single carrier with 10 7 second modulation symbol per second, for instance, 1,000 subcarriers modulated with only 10 4 modulation symbols per second can be transmitted. Considering a channel with a maximum delay ⁇ max of 10 ⁇ s, ISIs can thus easily be equalized or avoided.
  • the main advantage of OFDM is that no high-order bandpass filters with steep slopes are needed, as each orthogonal subcarrier can use the entire spectrum. With the aid of orthogonal subcarriers an easy separation of the individual subcarriers in the receiver is achievable.
  • the broadband radio channel is subdivided into a plurality of narrow-band subchannels (subcarriers) being independently modulated.
  • the signals are generated together with the aid of a Discrete Fourier Transform (DFT), and their spectra overlap mutually.
  • DFT Discrete Fourier Transform
  • Each subchannel only covers a small part of the total available frequency band, and as a consequence channel equalization becomes much simpler than in a single-carrier system. Instead of severely distorting several adjacent symbols, burst errors caused by fading only slightly distort a plurality of symbols.
  • the most important advantage of the OFDM transmission system arises in connection with frequency-selective channels.
  • the radio channel can approximately be deemed as frequency-nonselective.
  • the signal processing in the receiver can extremely be simplified, as the orthogonality of the OFDM subcarriers can be maintained during the whole transfer over the mobile radio channel.
  • just one complex transmission coefficient has to be estimated for each subcarrier.
  • the channel influence only consists in a multiplication with a complex transmission coefficient, aside from additive noise. Thereby, the object of a channel equalizer decreases to dividing each symbol by its respective transmission coefficient. In this way, a simple channel equalization is enabled, whereas possibly an equalization is not realizable if a single-carrier system with the same bandwidth is employed.
  • the channel equalization can easily be performed.
  • a coherent demodulation is enabled and, for instance, a Quadrature Amplitude Modulation (QAM) can be applied.
  • QAM Quadrature Amplitude Modulation
  • M-DPSK Differential Phase-Shift Keying
  • the (adaptive) equalization for coherent demodulation is simplified to a frequency-domain equalization comprising one complex multiplier per subcarrier in an appropriately designed OFDM system.
  • the multipath channel causes an attenuation and a phase rotation of each subcarrier amplitude.
  • phase-shift keying PSK
  • PSK phase-shift keying
  • not even an amplitude equalization is needed; it is enough to correct the phase variation.
  • differentially encoded modulation is used in the subcarriers, no channel estimation and correction is needed at all.
  • a channel estimation When using coherent modulation in the subcarriers, a channel estimation must be performed in the OFDM receiver.
  • the optimality of a two-dimensional Wiener filter to obtain estimates for the channel gains of a dispersive and time-variant channel is well-known in the state of the art. Thereby, the two dimensions need to be identified with the frequency-time grid spanned by the subsequent OFDM symbols in the frequency domain. Furthermore, so-called "pilot tones" are regularly multiplexed into the information-carrying OFDM symbols.
  • CP cyclic prefix
  • P preamble field
  • OFDM transmission systems are degraded by all influences which destroy said mutual subcarrier orthogonality. Hence, it is sensitive to carrier frequency offsets, and therefore, synchronization within a small fraction of a subcarrier spacing is crucial. Additionally, the OFDM symbol start needs to be identified in the receiver. Further negative effects like the degradation in a frame-offset or timing-jitter scenario and the influence of a non-synchronized sampling are analyzed in the relevant literature.
  • An OFDM signal is an ensemble of many narrowband subchannels. Therefore, the transmit signal of OFDM exhibits a high dynamic range and a large crest factor. Intermodulation as well as out-of-band power (spurious tones) are generated. Nonlinear devices like the power amplifier of the transmitter must be able to cope with the high dynamic range of the transmit signal to keep the out-of-band radiation under a certain limit imposed by the employed spectrum masks according to the underlying standard. This leads to an inefficient amplification and a large power consumption. Due to said effects many bit errors still occur - even at a high mean signal-to-noise ratio. For this reason, an efficient channel coding is needed for transmission systems using OFDM.
  • the energy received during the guard interval is wasted, as it is not used for the detection process of the receiver.
  • the maximum channel impulse response duration is long in comparison with the modulation interval, the number of carriers must be high to keep the loss in power efficiency due to the guard interval in reasonable limits.
  • An OFDM baseband signal s(t) consists of K equidistant subcarrier signals g k (t) (1 ⁇ k ⁇ K) having a subchannel spacing of ⁇ f.
  • the system bandwidth B sys is divided up into K equidistant subchannels.
  • a so-called "guard interval" with the length T G is prefixed to the signal g k (t) representing a periodic repetition of said signal g k (t).
  • Each of the K subcarrier signals g k (t) can be modulated with a complex modulation symbol S n,k , wherein the index n refers to the modulation interval and the index k indicates the subcarrier signal g k (t) in the neighboring OFDM block.
  • the OFDM baseband signal s(t) is sampled with a sampling rate of K ⁇ f ( ⁇ B)
  • a time-discrete signal corresponding to the Inverse Discrete Fourier Transform (IDFT) of S n,k the n-th transmitted modulation symbol of the k-th subcarrier g k (t) arises for the n-th OFDM block.
  • the time-discrete transmission signal can be generated from the sequence of the modulation symbols of all K subcarrier signals g k (t) by an IDFT which is realized as an Inverse Fast Fourier Transform (IFFT).
  • IFFT Inverse Fast Fourier Transform
  • the modulation symbols R n,k can be retrieved from the sampled receive signal in the receiver by means of a Fast Fourier transform (FFT).
  • FFT Fast Fourier transform
  • the transfer function H(f,t) of the channel can be regarded as quasi-stationary within the bandwidth ⁇ f of a subcarrier signal g k (t) and the duration T MC of a modulation symbol S n,k .
  • said OFDM subchannels can be modelled by means of Rayleigh channels or, with existing line of sight (LOS) connection, by means of Rice channels. These models imply an ideal interleaving, i.e. the transmission coefficients are uncorrelated in the latter case.
  • WSSUS Wide Sense Stationary Uncorrelated Scattering
  • the transmission of signals by means of an OFDM transmission system comprises the following steps:
  • pilot symbols which are possibly needed for the channel estimation and equalization must also be transmitted, dependent on the applied modulation technique. Obtained in this way, the digital baseband signal is then submitted to a D/A conversion, modulated to the carrier wave, amplified, and transmitted over the channel. In the receiver the complementary operations are applied in reverse order.
  • the receiver In order to assign an information to the received modulation symbols R n,k , which are attenuated and distorted in their phase as well as impaired by additive noise, the receiver must know the complex transmission coefficients H n,k . Demodulation techniques with said peculiarity are designated as "coherent" in technical language. Since the receiver can not ascertain the precise transmission coefficients H n,k , though, a channel estimation must be carried out instead which provides the estimated values ⁇ n,k for the transmission coefficients H n,k .
  • the receiver is able to retrieve the transmitted modulation symbol S n,k by means of simple threshold value decisions from R ⁇ n,k .
  • the receiver measures the transmission coefficients at that times and at those frequencies at which said pilot patterns were transmitted.
  • an interpolation which can preferably be realized as a filtering, yields the remaining transmission coefficients.
  • the transmission capacity of the system decreases due to the transmission of pilot symbols if said channel estimation is applied.
  • said pilot symbols are impaired by additive noise just as the remaining modulation symbols, so that the transfer function can not exactly be determined. The more pilot symbols are available, the more precisely the transfer function can be determined, however, the more data transmission capacity is wasted.
  • the distances of the pilot symbols should be n t modulation intervals and n f subcarrier signals at the most, with n t and n f fulfilling the inequations n t ⁇ 1 / 2 ⁇ f D , max ⁇ T MC or n t ⁇ 1 / 2 ⁇ ⁇ max ⁇ ⁇ f respectively, in which the herein used variables are defined as follows:
  • the interpolation can be realized by a two-dimensional filtering or by consecutive filtering in frequency and time direction.
  • the square brackets in the formula above round the value of its argument to the one higher integer value.
  • a single-shot channel estimation (based on a single multi-carrier pilot sequence) at the beginning of each data burst is not sufficient, as the radio channel exhibits a time-variant behavior due to the mobility of the users. If the frequency range of the carrier frequency is shifted from the 6-GHz band towards higher carrier frequencies, the degradation becomes worse.
  • the physical layer (PHY)
  • the encoding of the data is done by means of a punctured convolutional encoder having a code rate of 1/2 and a constraint length of 7.
  • the puncturing components are usually operated to result in a code rate of 3/4 or 9/16, respectively.
  • 4.Interleaving By means of a bit interleaving the efficiency of said convolutional encoder can be improved, and the influence of frequency-selective fading can be decreased.
  • the block length of the applied interleaver is equal to the number of the encodable bits per OFDM symbol.
  • the complex modulation symbols formed like that are called subcarrier modulation symbols.
  • the Data Link Control layer (DLC)
  • the object of the Data Link Control layer is to share the common used physical medium, the radio channel, among the mobile stations (MS) in a fair and efficient way.
  • a further object of said layer is to secure the transmission against errors occuring due to the transmission over the radio channel for each single connection.
  • the DLC layer is divided up into two sublayers called Medium Access Control (MAC) and Logical Link Control (LLC).
  • MAC Medium Access Control
  • LLC Logical Link Control
  • the MAC sublayer the physical channel is subdivided into time slots applying a TDMA technique.
  • the MAC sublayer is responsible for assigning transmission capacity to the respective mobile stations, dependent on their specific requirements.
  • the LLC sublayer provides an error correction by means of an Automatic Repeat Request (ARQ) protocol.
  • ARQ Automatic Repeat Request
  • the MAC sublayer divides the physical channel into frames of constant lengths.
  • a frame length of 2 ms is prescribed.
  • an OFDM symbol duration of 4 s is given, this corresponds to 500 OFDM symbols per frame.
  • the access to the common used physical channel is coordinated by a central instance, the Access Point (AP) or the Central Controller (CC), respectively. It dynamically assignes transmission capacity in the form of time slots. Therefore, the AP needs information about the number and the service class of the data packets in the mobile terminals (MT) waiting for being transmitted.
  • AP Access Point
  • CC Central Controller
  • transport channels are offered by the physical layer (PHY).
  • PHY physical layer
  • BCH Broadcast Channel
  • FCH Frame Channel
  • the parameters of the transport channels according to the ETSI-BRAN standard can be taken from the following table: Physical Channel Direction of the transmission Length [Byte] Access Feedback Channel (ACH) Downlink 9 Broadcast Channel (BCH) Downlink 15 Frame Channel (FCH) Downlink n ⁇ 27 Long Transport Channel (LCH) Down-/Uplink 54 Short Transport Channel (SCH) Down-/Uplink 9 Random Access Channel (RCH) Uplink 9
  • BCH Broadcast Channel
  • FCH Frame Channel
  • LCH Long Transport Channel
  • SCH Short Transport Channel
  • RCH Random Access Channel
  • the HIPERLAN/2 standard defines two PHY bursts, as described above, each for the downlink and for the uplink, respectively, and an optional Direct Link burst which shall not be considered here.
  • PDU Protocol Data Unit
  • DLC Data Link Control layer
  • the format of the herein applied PHY bursts, including said Direct Link burst, can be taken from the following figure: a) Broadcast burst Section 1 Section 2 Section 3 Payload ⁇ 4 ⁇ s ⁇ ⁇ 4 ⁇ s ⁇ ⁇ 8 ⁇ s ⁇ ⁇ N ST ⁇ 4 ⁇ s ⁇ A R A A R R A B A B B I B CP C C CP d Data 1 b) Uplink burst with long preamble Direct Link burst Section 2a Section 3 Payload ⁇ 8 ⁇ s ⁇ ⁇ 8 ⁇ s ⁇ ⁇ N ST ⁇ 4 ⁇ s ⁇ B B B B B B B B B B B B B B B B B B B B B B B B B B B B B B B B B B B B B B B B B B B B B B I B CP C C CP d Data 1 c) Uplink burst with short preamble Section 2 Section 3 Payload ⁇ 4 ⁇ s ⁇ ⁇ 8 ⁇ s ⁇ ⁇ N
  • Section 1 and 2 of the broadcast burst are designed for the identification of the MAC frame start, whereas section 3 is designed to be used for a channel estimation.
  • Section 2 of the uplink burst is also.designed to enable a successful automatic gain control (AGC) adjustment for all channels. Further details can be taken from the HIPERLAN/2 specification.
  • the data part, called payload contains the OFDM symbols being transmitted on fixed subcarriers. Thereby, four pilot symbols are multiplexed into each payload OFDM symbol. Typically, these pilot symbols are used for frequency tracking purposes. Since they are allocated to fixed subcarriers and usually do not fulfil the sampling theorem in frequency direction, they can not be used for channel tracking.
  • sytems data packets are transmitted comprising a so-called preamble (which consists of OFDM symbols) followed by data OFDM symbols.
  • a QPSK modulation with a code rate of 3/4 is applied, the length of the total burst (comprising said preamble and said data) can, for instance, vary between 16 ⁇ s (preamble length: 8 ⁇ s, data length: 8 ⁇ s, if only one SCH is applied) and 1,928 ⁇ s (preamble length: 8 ⁇ s, data length: 1,920 ⁇ s, if 80 SCHs are applied).
  • said extension field (CP) is composed of a cyclic repetition of said OFDM symbols (P 2 , P 3 ) comprising a copy of the last 32 samples of one of the two OFDM symbols (P 2 or P 3 ). It is thus double in length compared to the cyclic prefix (CP d ) of normal data symbols.
  • the entire burst is formed by concatenating the above-described preamble (P ⁇ P 1 ) with the data payload comprising the data OFDM symbols, and possibly with other preambles (P 2 , P 3 , ...) as depicted below: P D D D D D D D D D D D D D D D D D D D D D D D D D 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
  • the scrambling of pilot patterns of any kind is applied to randomize the deterministic behavior of predefined pilots in order to minimize deterministic spectral peaks.
  • Each sequence element is used for a single OFDM symbol.
  • the scrambler defined in that way should be initialized at the beginning of each PDU train.
  • the continuous pilot pattern C l,n is not used for a channel estimation with the purpose to equalize the channel, but e.g. for the purpose of a phase tracking.
  • OFDM demodulation consists of three steps:
  • a broadcast burst consists of a preamble followed by a payload section of N successive data OFDM symbols.
  • the preamble starts with two sections containing different kinds of OFDM symbols, and ends with a third section consisting of two OFDM symbols (C) of normal length preceded by a cyclic prefix (CP) of the symbols, the cyclic prefix CP being a copy of the 32 last samples of the C symbols.
  • C OFDM symbols
  • CP cyclic prefix
  • the structure of a data burst contains a first preamble field, a second signal field, and a third data field.
  • the first preamble field is used for synchronisation and consists of ten short symbols and two long symbols.
  • the two long symbols which are dedicated to channel and fine frequency offset estimation, are preceded by a cyclic extension.
  • the second signal field contains rate and length information.
  • the third data field consists in series of OFDM symbols related to data, each OFDM symbols being preceded by a cyclic extension.
  • each of the applied pilot pattern based channel estimation and/or channel tracking techniques is optimized to a specific purpose, and thus it contains certain limitations.
  • HIPERLAN/2 no channel tracking is performed.
  • the channel transfer function is estimated during the preamble part of the data burst, and remains fixed during the entire burst ("single-shot channel estimation") to support a coherent demodulation of the received signal. Any channel variations during the reception of the burst, for example caused by the mobility of the users, are not tracked. Therefore, a degradation of the data transmission for any time-variant channels must be expected. Additionally, as there is no update of said single-shot channel estimation, a poor quality channel estimation at the beginning reduces the reception quality of the whole burst.
  • pilot pattern applied to the DVB-T standard is an optimized solution for time-variant channels. Nevertheless, a significant disadvantage of this pilot pattern is the complexity of its implementation, for example concerning the required processing power for the interpolation, and the buffers needed for storing multiple complete OFDM symbols.
  • the implementation complexity is also a fundamental disadvantage of pilot patterns applied to the DFE standard, likewise concerning the required processing power.
  • the DFE equalizer needs valid channel state information as well (as starting point), and thus a proper pilot pattern is required.
  • Channel tracking can also be performed by applying a Decision Feedback Equalization (DFE).
  • DFE Decision Feedback Equalization
  • the usage of DFE is restricted to the lower level modulation schemes (e.g. BPSK and QPSK), whereas the channel tracking is unsatisfactory for other modulation schemes (e.g. 16QAM and 64QAM).
  • the usage of a DFE implies a significant increase in the complexity of the applied receiver architecture.
  • the underlying invention describes a low-complexity and low-effort solution for a pilot pattern of a pilot-assisted high-speed wireless multi-carrier system used for supporting mobile applications within environments being severely impaired by a time-varying multipath fading behavior of the mobile radio channel.
  • the invention is specially optimized for extending conventional "single-shot" pilot pattern based channel estimation schemes to cope with said severe channel conditions, and can easily be realized. Additionally, the proposed scheme allows a continuous improvement of a single-shot channel estimation.
  • the existing preamble (or parts of said preamble) is multiplexed into the OFDM symbol stream of a associated data burst to support channel tracking in the receiver.
  • the already defined preamble is used, since it implies the advantage of an easy implementation without changing the existing channel estimation algorithm.
  • the guard interval is inserted to suppress ISI caused by multipath distortion.
  • Said guard interval is also called cyclic prefix (CP). It comprises a copy of the last part of the OFDM symbol which is placed in front of the transmitted symbol. This makes the transmitted symbol periodic, which plays a decisive role in correctly identifying frames to avoid ISI and ICI.
  • the price one has to pay for this advantage is a loss of signal-to-noise ratio, as the cyclic prefix is subjected to the "noisy" channel just as the signal is concerned, and by occupying signal space less power is provided for the transmission of the signal frame.
  • a filtering function is applied in order to reduce the out-of-band radiation.
  • the signal is submitted to a D/A conversion to produce the analog baseband signal, upconverted to the radio frequency (RF), and finally transmitted.
  • RF radio frequency
  • a data burst structure comprising a first preamble and an OFDM symbol stream
  • the first preamble comprises a cyclic prefix for the synchronization of a transmitter and a receiver of said data burst structure, and at least one pilot symbol for performing a channel estimation
  • at least one additional preamble is multiplexed into the OFDM symbol stream, the at least one additional preamble comprising at least one pilot symbol for performing a channel estimation.
  • the at least one additional preamble can be multiplexed after each N-th OFDM data symbol, in which N is defined as 2 M , M being a positive integer value.
  • the at least one additional preamble can correspond to the first preamble, or alternatively can be different from the first preamble.
  • Each of the at least one additional preamble can comprise a plurality of pilot symbols for performing the estimation of a plurality of channels related to a plurality of transmitting antennas.
  • At least two additional preambles can be inserted into the OFDM symbol stream, each of the at least two additional preambles comprising at least one pilot symbol for performing a channel estimation related to a different transmitting antenna.
  • the cyclic prefix can comprise a part of the at least one pilot symbol.
  • the at least one pilot symbol of the first preamble and the at least one pilot symbol of the at least one additional preamble can be scrambled by means of a pseudo-noise scrambling sequence generated by a generator polynomial.
  • the data burst structure can further be adapted to enable a receiver to steadily exploit channel estimates during the reception of the data burst by weighting previously obtained channel estimates in order to reduce the effect of noise on the channel estimation.
  • a mobile transmitter comprising means for generating and transmitting a signal having a data burst structure as previously defined.
  • a mobile receiver comprising means for receiving and demodulating a signal having a data burst structure as previously defined, and means for performing a channel estimation on the basis of the at least one pilot symbol of the at least one additional preamble of said signal.
  • the mobile receiver can comprise means for performing a channel equalization and can further comprise means for performing a synchronization on the basis of the cyclic prefix of said signal.
  • a mobile transmission system comprising at least one mobile transmitter as previously defined and at least one mobile receiver as previously defined.
  • a method for supporting wireless communication over a mobile radio channel by means of pilot-assisted wireless multi-carrier systems comprises the steps of performing an OFDM multi-carrier modulation and an OFDM multi-carrier demodulation of a signal having a data burst structure as previously defined, performing a channel estimation on the basis of the at least one pilot symbol of the at least one additional preamble of said signal.
  • a channel equalization can be performed. Additionally, a synchronization can be performed on the basis of the cyclic prefix of said signal.
  • Figure 1b shows the unextended OFDM symbol stream 106; the extended OFDM symbol stream 107 comprising data symbols 107a-d and multiplexed reference symbols 105 can be taken from figure 1c.
  • the multiplexing of the modified preamble (P') field 206 remains the same as described above for the unchanged preamble (P) field 201, which is again depicted in figure 2a.
  • Figure 2c shows the unextended OFDM symbol stream 211; the extended OFDM symbol stream 212 comprising data symbols 212a-d and multiplexed reference symbols 209 and 210, respectively, can be seen in figure 2d.
  • the main object of an applied pilot scrambling sequence is to randomize (to "whiten") the transmitted signal.
  • it is proposed to maintain said scrambling even during the transmission of the additional reference symbol. That means that from the pilot scrambler point of view the multiplexing of additional reference symbols is transparent.
  • the additional reference symbols it is proposed to multiply the complete additional reference symbol (and not only the carriers at the four pilot positions) with the actual scrambling value SC n ⁇ ⁇ -1, +1 ⁇ . Furthermore, as an additional advantage, said multiplication serves to randomize the spectrum of the signal to be transmitted.
  • the multiplication of the additional reference symbol can either be performed in the time domain by multiplying the complex samples with either '+1' or '-1', or in the frequency (subcarrier) domain by multiplying the complex subcarrier information of the additional reference symbol information with either '+1' or '-1' as shown in the figures 3c and 3d, respectively.
  • Figure 3a shows again the structure of the unchanged preamble (P) field 301, whereas figure 3b exhibits the structure of the modified preamble (P') field 306.
  • the other components 309-312d depicted in the figures 3b and 3c are identical with the above-described components 209-212d depicted in the figures 2b and 2c.
  • channel estimation is needed for each separate transmitting antenna. This is applicable to schemes like Space-Time Block Coding (STC), which uses multiple transmitting antennas.
  • STC Space-Time Block Coding
  • the symbol 'P' refers to the sequence of channel estimation information (P 1 ...P ⁇ ...P m ) obtained from each of the m antennas.
  • An other multiplexing option is, for instance, alternatively multiplexing channel estimation symbols from one or the other transmitting antenna into the OFDM symbol stream 407a-f as shown in figure 4c.
  • channel estimates 507a-d are steadily exploited during the reception of the extended OFDM symbol stream comprising data symbols 508a-d and multiplexed reference symbols 501 and 505, respectively, by weighting previously obtained channel estimates 507a-d in order to reduce the effect of noise on the channel estimation.
  • the employed weights are typically powers of a number within the range of 0 to 1.
  • Figure 6 refers to a block diagram for the employed mobile transmission system comprising one transmitter 601 and one receiver 602 for supporting wireless communication over a mobile radio channel by means of a pilot-assisted wireless multi-carrier system considering channel estimation and equalization.
  • the input is a binary data stream 604.
  • a scrambler 605 is used for randomizing the transmitted input data bits 604.
  • the data is then transformed into a multilevel signal to be prepared for an OFDM modulation 609.
  • the serial data stream is converted to parallel by means of the serial-parallel converter 609a, the data rate gets reduced by N, where N is the number of parallel subchannels used for the applied OFDM modulation.
  • N is the number of parallel subchannels used for the applied OFDM modulation.
  • these parallel channels are essentially low data rate channels, and since they are narrow-band channels they experience flat fading. This is the greatest advantage of the applied OFDM technique.
  • the data symbols are mapped to the subcarriers using an Inverse Fast Fourier Transform (IFFT) performed by a digital signal processor 609b, and reconverted to serial by means of a parallel-serial converter 609c.
  • IFFT Inverse Fast Fourier Transform
  • a guard interval is inserted with the aid of the guard interval insertion unit 610.
  • CP cyclic prefix
  • a signal filtering is performed in order to reduce the out-of-band radiation. Therefore, a raised-cosine window function can be employed.
  • the signal is then submitted to a digital-analog converter 611 to produce the analog baseband signal, modulated to the RF carrier wave by means of an RF upconversion unit 612a, amplified, and transmitted over the mobile radio channel 603.
  • the mobile radio channel 603 is assumed to be an Additive White Gaussian Noise (AWGN) channel.
  • AWGN Additive White Gaussian Noise
  • the complementary operations are applied in reverse order.
  • the received RF signal is downconverted to the baseband by means of the RF downconverter 614b, and submitted to an analog-digital converter 615.
  • the guard interval is removed with the aid of the guard interval removal unit 616, the OFDM demodulation 617 can be performed.
  • the data is then transformed into a multilevel signal to be prepared for an OFDM demodulation.
  • the serial data stream is converted to parallel by means of the serial-parallel converter 617a, all subcarriers are separated by applying a Fast Fourier Transform (FFT) performed by a digital signal processor 617b, and reconverted to serial by means of a parallel-serial converter 617c.
  • FFT Fast Fourier Transform
  • the data stream is submitted to a symbol-to-bit demapper 620 followed by a deinterleaver 621, a decoder 622, and a descrambler 623 to obtain the output data bits 624.
  • Figure 7 exhibits a detailed view of the OFDM modulation 701 performed in the transmitter with multiplexing data symbols and pilot symbols, and the OFDM demodulation 702 performed in the receiver.
  • IFFT Inverse Fast Fourier Transform
  • FFT Fast Fourier Transform
  • a convolutional encoder 607 interleaver 608 bit-to-symbol mapper 609 OFDM modulator 609a serial-parallel (S/P) converter 609b digital signal processor performing an Inverse Fast Fourier Transform (IFFT) 609c parallel-serial (P/S) converter 610 guard interval insertion (GII) and windowing in order to reduce the out-of-band radiation 611 digital-to-analog (D/A) converter 612a RF upconversion unit 612b transmitting antenna 613 additive noise 614a receiving antenna 614b RF downconversion unit 615 analog-to-digital (A/D) converter 616 guard interval removal (GIR) and windowing in order to reduce the out-of-band radiation 617 OFDM demodulator 617a serial-parallel (S/P) converter 617b digital signal processor performing a Fast Fourier Transform (FFT) 617c parallel-serial (P/S) converter 618 channel estimator 619 channel equalizator 620 symbol

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  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Power Engineering (AREA)
  • Mobile Radio Communication Systems (AREA)

Claims (17)

  1. Structure de salves de données comprenant un premier préambule (209) et un train de symboles OFDM (211),
    dans lequel le premier préambule (209) comprend :
    - un préfixe cyclique (202) pour la synchronisation d'un émetteur (601) et d'un récepteur (602) de ladite structure de salves de données ; et
    - au moins un symbole de pilote (203, 204) pour réaliser une estimation de canal,
    caractérisée en ce qu'elle comprend au moins un préambule additionnel (210) dans le train de symboles OFDM (211), l'au moins un préambule additionnel (210) comprenant au moins un symbole de pilote (208) pour réaliser une estimation de canal.
  2. Structure de salves de données selon la revendication 1, comprenant l'au moins un préambule additionnel (210) après chaque N-ième symbole de données OFDM, où N est défini en tant que 2M, M étant une valeur entière positive.
  3. Structure de salves de données selon la revendication 1 ou 2, dans laquelle l'au moins un préambule additionnel (210) correspond au premier préambule (209).
  4. Structure de salves de données selon la revendication 1 ou 2, dans laquelle l'au moins un préambule additionnel (210) est différent du premier préambule (209).
  5. Structure de salves de données selon la revendication 1 ou 2, dans laquelle chacun des au moins un préambule additionnel (210) comprend une pluralité de symboles de pilote (402, 403) pour réaliser l'estimation d'une pluralité de canaux rapportés à une pluralité d'antennes d'émission.
  6. Structure de salves de données selon la revendication 1 ou 2, dans laquelle au moins deux préambules additionnels (210) sont insérés dans le train de symboles OFDM (211), chacun des au moins deux préambules additionnels (210) comprenant au moins un symbole de pilote (402, 403) pour réaliser une estimation de canal rapportée à une antenne d'émission différente.
  7. Structure de salves de données selon l'une quelconque des revendications précédentes, dans laquelle le préfixe cyclique (202) comprend une partie de l'au moins un symbole de pilote (203, 204).
  8. Structure de salves de données selon l'une quelconque des revendications précédentes, dans laquelle l'au moins un symbole de pilote (203, 204) du premier préambule (209) et l'au moins un symbole de pilote (208) de l'au moins un préambule additionnel (210) sont brouillés au moyen d'une séquence de brouillage de pseudo-bruit qui est générée par un polynôme de générateur.
  9. Structure de salves de données selon l'une quelconque des revendications précédentes, adaptée pour permettre qu'un récepteur exploite en régime établi des estimations de canal pendant la réception de la salve de données en pondérant des estimations de canal obtenues au préalable afin de réduire l'effet du bruit sur l'estimation de canal.
  10. Emetteur de mobile comprenant un moyen (609, 612b) pour générer et émettre un signal présentant une structure de salves de données selon l'une quelconque des revendications 1 à 9.
  11. Récepteur de mobile comprenant :
    - un moyen (614a, 617) pour recevoir et démoduler un signal qui présente une structure de salves de données selon l'une quelconque des revendications 1 à 9 ; et
    - un moyen (618) pour réaliser une estimation de canal sur la base de l'au moins un symbole de pilote (208) de l'au moins un préambule additionnel (210) dudit signal.
  12. Récepteur de mobile selon la revendication 11, comprenant un moyen (619) pour réaliser une égalisation de canal.
  13. Récepteur de mobile selon la revendication 11 ou 12, comprenant un moyen pour réaliser une synchronisation sur la base du préfixe cyclique (202) dudit signal.
  14. Système de transmission de mobile comprenant au moins un émetteur de mobile (601) selon la revendication 10 et au moins un récepteur de mobile (602) selon l'une quelconque des revendications 11 à 13.
  15. Procédé pour supporter une communication sans fil sur un canal radio mobile (603) au moyen de systèmes multiporteuses sans fil assistés par pilote, comprenant les étapes de :
    - réalisation d'une modulation multiporteuse OFDM (609) et d'une démodulation multiporteuse OFDM (617) d'un signal présentant une structure de salves de données selon l'une quelconque des revendications 1 à 9 ;
    - réalisation d'une estimation de canal (618) sur la base de l'au moins un symbole de pilote (208) de l'au moins un préambule additionnel (210) dudit signal.
  16. Procédé selon la revendication 15, dans lequel une égalisation de canal est réalisée.
  17. Procédé selon la revendication 15 ou 16, dans lequel une synchronisation est réalisée sur la base du préfixe cyclique (202) dudit signal.
EP01116839A 2001-07-10 2001-07-10 Symboles de référence pour l'estimation de canal pour la transmission multiporteuse Expired - Lifetime EP1276288B1 (fr)

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EP01116839A EP1276288B1 (fr) 2001-07-10 2001-07-10 Symboles de référence pour l'estimation de canal pour la transmission multiporteuse
ES01116839T ES2278661T3 (es) 2001-07-10 2001-07-10 Simbolos de referencia para la estimacion de canales con transmision multiportadora.
DE60125925T DE60125925T2 (de) 2001-07-10 2001-07-10 Referenzsymbole zur Kanalschätzung bei Mehrträgerübertragung

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US8848781B2 (en) 2007-05-16 2014-09-30 Thomson Licensing Apparatus and method for encoding and decoding signals
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US8848781B2 (en) 2007-05-16 2014-09-30 Thomson Licensing Apparatus and method for encoding and decoding signals
US8873620B2 (en) 2007-05-16 2014-10-28 Thomson Licensing Apparatus and method for encoding and decoding signals
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EP1276288A1 (fr) 2003-01-15
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